Coating materials, e.g. polymers, are often used as protective barriers on circuit boards, electrical components, medical devices and the like. Parylene is a generic term often used to describe a class of poly-p-xylylenes (also sometimes referred to in the art as para-xylylenes) which are derived from a dimer of the structure: ##STR1## where X is typically hydrogen or a halogen. Due to its ability to provide thin films and conform to substrates of varied geometric shapes, parylene is ideally suited for use as a conformal coating.
Typically, parylene is applied by vapor deposition wherein the parylene monomer is condensed and polymerized directly on the article to be coated. Since the parylene monomer is not stable, the parylene dimer, as illustrated above, is used as the starting material.
The basic steps in applying parylene coatings by vapor deposition are disclosed, for example, in U.S. Pat. No. 4,945,856, issued Aug. 7th 1990. At col. 1, lines 32 to 66 it is disclosed that:
Accordingly, in most commercial applications, paraxylylene polymers are deposited on desired substrates by a pyrolytic deposition process known specifically as the "parylene process." Such process begins with the vaporization of a cyclic di-para-xylylene dimer. The dimer is pyrolytically cleaved at temperatures of about 400.degree. to 750.degree. C. to form a reactive para-xylylene monomer vapor. Thereafter, the reactive monomer vapor is transferred to a deposition chamber wherein the desired substrates are located. Within the deposition chamber, the reactive monomer vapor condenses upon the desired substrates to form a para-xylylene polymer or co-polymer film. PA1 Any monomer vapor which fails to condense within the deposition chamber is subsequently removed by a cold trap which is maintained at approximately -70.degree. C. PA1 The entire parylene process is generally carried out in a closed system under constant negative pressure. Such closed system may incorporate separate chambers for the (a) vaporization, (b) pyrolysis, and (c) deposition steps of the process, with such chambers being connected by way of appropriate plumbing or tubular connections. PA1 . . . a condensation coating deposition chamber wherein uniformity of monomer vapor is maintained by inducting a rotary flow pattern within the chamber. Such rotary flow pattern obviates the need for baffles or other hardware elements thereby lessening the amount of polymer wasted during the process. Additionally, the parylene deposition chamber of the present invention provides greater versatility than the prior art devices because it is of modular design and, thus, easily detachable from the pyrolytic generating unit. PA1 a tank-like chamber body having a floor, a cylindrical wall, and an openable and closable lid. The monomer vapor enters tangentially near the top of the chamber through a tangentially connected monomer supply line. Such tangential entry of the monomer vapors results in a generally annular rotational flow of the vapors as they descend axially through the inner confines of the deposition chamber. PA1 (a) a deposition chamber having: PA1 (b) means for creating a vacuum in said deposition chamber by applying a suction to the first opening in said base; and PA1 (c) means for introducing the coating material through the second opening in said base into said deposition chamber. PA1 (a) a deposition chamber having: PA1 (b) a vacuum pump connected to the bottom side of said base and in communication with the first opening in said base; PA1 (c) a heated pyrolysis zone connected to the bottom side of said base and in communication with the second opening in said base and positioned such that parylene monomer can pass upwardly through the pyrolysis zone in a generally vertical direction and be introduced into the deposition chamber to create a generally upward flow pattern; and PA1 (d) a heated vaporization zone connected to said pyrolysis zone and positioned such that vaporized parylene dimer can be introduced into said pyrolysis zone. PA1 (a) a deposition chamber; PA1 (b) means for creating a vacuum in said deposition chamber; and PA1 (c) means for introducing the coating material into said deposition chamber: the improvement which comprises means for introducing parylene monomer into the deposition chamber to create a generally upward flow pattern of the parylene monomer in the deposition chamber.
A typical problem in the parylene process is that parylene coats the inside of the deposition chamber and all of the objects within the deposition chamber as well as the articles which are desired to be coated. Thus, a substantial amount of parylene can be wasted.
The above referenced patent proposes a solution to the problem of wasting parylene. As stated at col. 2, lines 57 to 67, the patentee's invention provides:
However, in practice, rotary flow patterns can be difficult to establish at the low pressures of the parylene process. As a result, coating thicknesses can vary drastically throughout the deposition chamber.
At col. 3, lines 3 to 12 the patentee discloses that the deposition chamber comprises:
An improved solution to the problem of parylene waste is desired which does not require the elimination of baffles and the rather complicated flow arrangement described in the above-referenced patent to compensate for the lack of baffles. Further, in recognition that the surface area of the articles to be coated varies with the geometry and number of the articles, it would be desirable to provide a vapor deposition coating apparatus wherein the surface area of the deposition chamber could easily be modified to accommodate the surface area of the articles to be coated.